In particular, if either MATLAB or Octave is missing, one needs to pass either
--disable-matlab or --disable-octave.
Moreover, several new configure flags have been introduced for disabling some
components:
--disable-doc
--disable-dynare++
--disable-mex-dynare++
--disable-mex-ms-sbvar
--disable-mex-kalman-steady-state

- integrate with autoconf/automake, so that "make html pdf" now build the
manual
- remove the old texinfo manual
- the PDF manual is now called dynare-manual.pdf (Closes #1543)
- add a Gitlab CI job that updates the online unstable manual under
www.dynare.org/manual-unstable/